This invention relates to color video CRT (Cathode Ray Tube) display systems, including Eidophor type light-valve projector systems and projection display systems using CRT light sources. More particularly, the invention relates to data converters for color video CRT display systems in which digitized color picture information is converted to provide analog color signals for control of the video amplifiers in a CRT display.
In a color video CRT display system, operation involves formation and control of one or more electron beams. The beam or beams can be focused to a desired cross section and varied in position and intensity to produce a visible or otherwise detectable pattern on a screen. Each beam is conventionally generated by an electron gun driven with an amplified video voltage signal applied across gun electrodes. The intensities of the beam and resultant emissions of light usually vary directly in proportion to the level of the applied voltage. In state of the art systems, the beam is deflected and focused by electrostatic or magnetic means.
Most color systems employ three electron beams, a separate beam being used for causing emission of each of the three primary additive video colors of light. These are the colors red, green and blue, which when mixed in proper proportions, will form any other color or colors desired. For example, mixture of the three primary colors in equal proportions forms white. Black is formed by the absence of the three primary colors in any proportion.
In color video CRT display systems, pictures are generally formed from color picture information which is provided by a data source, such as a video broadcasting station, a video camera or a computer. The color picture information includes color information for controlling the intensity of the electron beam or beams which are produced in the CRT display. It also includes deflection information for controlling the deflection or target location of each beam. The deflection information may consist of horizontal and vertical sync information, vector coordinate information, or order of storage of color information in a memory, depending upon the type of system involved.
Data converters are used as necessary in color video CRT display systems to decode color picture information from the data source and to convert the information to a form required for driving the CRT display. A color CRT display normally requires input of color information in the form of three separate serial analog data or color signals, where each such signal controls the electron beam intensity for a different one of the three primary additive video colors. These analog color signals are usually voltage signals characterized by an "off" or "black" signal voltage level and an "on" signal voltage level. In some systems, the "on" signal voltage level may be variable for color control or control of intensity variations in the color picture formed on the CRT display which is used.
Most color video CRT displays require input of a low-level analog color signals having peak-to-peak voltages on the order of about 0.5 to 5 volts. These low-level analog color signals conventionally control or are input to video amplifiers in the CRT display unit. The video amplifiers provide amplified or high-level analog color output signals having peak-to-peak voltages on the order of about 30 to 150 volts as required for driving an electron gun in the particular CRT display.
In some data converters for color video CRT display systems, color picture information is processed or decoded to a digitized form. Examples are data converters which have semiconductor logic circuits for information processing or semiconductor memories for screen refresh purposes. Such data converters commonly provide color information in the form of separate serial digital data signals for each of the three primary additive video colors. These digital color signals may be used directly as the low-level color analog signals for input to the CRT display unit, or they may be used as inputs to a gain control and signal conversion stage for which the low-level analog color signals are output. The gain control and signal conversion stage may be used to provide conversion from digital to required analog voltage levels, and to provide low-level analog color signals with requisite current sourcing capability for driving the CRT display.
Current technology has used special high-speed, high-cost switching transistors, or differential amplifiers, to drive large band width (e.g., 25 MHz band width) color video amplifiers. In addition, complicated circuitry has been often seen as necessary for providing variable low-level analog color signals for controlling video brightness or contrast. Some circuits fail to provide precise color or voltage tracking because of temperature effects on active component circuit parts.
It was against the foregoing background that the present invention was made.